io_uring/rsrc: clean up __io_sqe_buffers_update()

Inline offset variable, so we don't use it without subjecting it to
array_index_nospec() first.

Signed-off-by: Pavel Begunkov <asml.silence@gmail.com>
Link: https://lore.kernel.org/r/77936d9ed23755588810c5eafcea7e1c3b90e3cd.1681395792.git.asml.silence@gmail.com


Signed-off-by: Jens Axboe <axboe@kernel.dk>